Tesla wins high ratings in collision tests, but reportedly committed errors in its armor design

When it comes to Bayerische Motoren Werke (BMW) AG's (ETR:BMW) electric i3 Coupe, interest in the vehicle, coupled with BMW's decision to boldly put the Coupe out in the hands of consumers for testing has led to a great deal of press -- some of it mixed.

When it comes to safety, the vehicle's credentials have been called into question. Some have questioned whether EVs in general, much less the i3 Coupe, are safe.

I. Battery Fires and the Need for Armor

After my CES test drive of the BMW all-electric entry-level luxury vehicle, I had a deep discussion with BMW engineers and sales staff. This discussion cast some light into the vehicles design, and why BMW is convinced that it has designed a safer EV.

Tesla Motors Inc. (TSLA) learned that lesson the hard way. While its Models S earned industry leading crash-test scores, it was plagued by at least three recent battery fire incidents. Some of these incidents boiled down to user error after drivers hit objects, penetrating the battery housing and damaging the cell. The vehicle informed the users that they shouldn't drive, but they did anyways -- and it let them.

Tesla Model S

But some culpability does lie with Tesla as well. Specifically it equipped a vehicle with very low clearance with only a quarter-inch (6 mm) thick aluminum panel over the batteries, which lie along the floor in the middle of the vehicle. One our regular readers, Reclaimer77 pointed this flaw out, commenting:

Depends on what the metal in question is. In this case Tesla opted for aluminum.

So when the ~5,000 pound Model S runs over something, and a thin soft piece of aluminum is all that stands between all that force and the batteries - it's no wonder.

Aluminum is just a very poor choice for such an application. I understand the need to save weight, but come on.... Musk needs to do a recall and replace every aluminum plate with a steel one.

A Model S is seen here after its weak aluminum plate is penetrated by debris.
[Image Source: Petrus Breedt]

The "old rule of thumb" is that steel is twice as strong, but three times as heavy as aluminum.

II. The Cost of Using Weaker Aluminum Alloys

Alloys can improve the elastic modulus of aluminum, fighting penetration. A clue to what alloy Tesla might have used comes in publicly released comments that 25 tons of force were required to punch a 3 inch hole.

Objects have only been able to punch through because Tesla used a cheap, light aluminum plate. [Image Source: NC State/Lattice Energy LLC]

So we know that the tensile strength of Tesla's armor plate was at most 146 MPa -- a strength that suggests that it is not steel. Tesla later acknowledged that it used aluminum in the plate, but the question remained what alloy was used. Some alloys are almost as strong as steel -- others not so much. Tesla's armor plate seemed to fall under the "not so much" category.

Only a thin sheet of aluminum stands between the battery pack and the road in the Model S.
[Image Source: GM-Volt]

A publication [PDF] in AluReport casts further light on the metal used in the armor plate:

Now resistance spot welding has joined the rank of large-scale aluminium joining technologies already mentioned: DeltaSpot® by Fronius...

Tesla Motors also uses the state-of-the-art DeltaSpot® resistance spot welding process in its Limousine Model S to produce a large number of two-and three-layered joints between aluminium sheets, sections and castings [4].

All of the large-scale application mentioned above, however, relate to established AlMg and AlMgSi wrought alloys and AlSiMg die-casting alloys [4].

So this tells us Tesla used a wrought aluminum plate of an aluminum alloy with magnesium or magnesium+silicon. Looking at various alloy fact sheets [PDF], and knowing that the Tesla plate has a tensile strength of under 146 MPa, it's clear that the automaker didn't even use the strongest available alloys (which likely were more expensive).

An analysis by Lattice Energy LLC suggests that had Tesla used ferritic stainless steel, instead of a low-cost aluminum alloy, it would have been able to withstand up to 71 tons of impact force on the plate without breaking. And that's not to mention that even stronger aluminum alloys have lower fatigue strength, meaning over time it will be weakened to a greater degree from repeated non-penetrating blows.

There are alternatives -- such as using a thinner plate, but backed by an aluminum space frame (a technique that could produce a stronger sheet), or using more expensive alloy blends.

It'd be misleading to say cost was the only reason Tesla made this choice. As mentioned, aluminum may be only a third as strong (roughly) as steel, but it's also half the weight. Switch to steel would likely add around 800 pounds to the vehicle weight, reducing the range and performance. But even if you make that argument, it seems foolish to trade safety for performance in a mass market vehicle.

Tesla chose cost, speed, over safety. [Image Source: AP]

Factor in that more expensive alloys could have deliver steel-like strength with the weight advantages of aluminum, and you come to see that Tesla indeed threw caution to the wind and chose a cheap option. Now it's paying the consequences.

Tesla's choice is all the more baffling in that it otherwise designed a vehicle that has great frame integrity, putting up incredible numbers in various crash tests. It'd be like painting the Mona Lisa, only to scribble a pine tree next to her in quill pen. Tesla deserves praise for the safety of the ovearall design, but rebuke for its choice of armor plate.

While the i3 Coupe doesn't really need to be as damage resistant due to its substantiall higher clearance than the Model S, it goes the extra mile, where Tesla chose the cheap route. Namely, BMW chose to put a quarter-inch thick high-strength steel armor plate over its batteries.

For those who skipped Eng. 101, here's a demonstration of why steel is a better choice than aluminum for armor applications. [Image Source: Lorenz Industries]

You won't see this on any crash test under current standards (which regulators may look to revise after the rash of recent battery fires from multiple manufacturers), but by better safeguarding the battery pack from damage, the i3 Coupe is indeed in some ways safer than Tesla's Model S.

IV. Debunking the Mini Myth

BMW also wanted us to debunk some of the misinformation that some confused readers were spreading on the original blog. Specifically, one common misconception among many people is that the i3 Coupe is "built on the Mini platform."

One reader wrote:

It's a Mini with an electric motor.

Another wrote:

Yeah. You're off to a bad start when you base it around a Mini platform.

The confusion is understandable. BMW does own the Mini nameplate. And unsurprisingly some recent editions of the Mini Cooper (e.g. the Paceman) shares some style cues with the i3 Coupe. And it's true the drivetrain is a descendant of the Mini E -- the electric Mini variant that was sold in 2008.

But the Mini E and BMW i3 Coupe do not use the same drivetrain. BMW in fact has a had three major drivetrain iterations, since 2008.

V. The Evolution of the EV at BMW

BMW's first drivetrain was found on the Mini E -- essentially a Mini Cooper equipped with a mod kit. While the Mini E was an "official" BMW, fans had been making their own electrified Mini Coopers by several years at that point.

Notably it introduced the under-the-seat style flat battery pack which is also used by Tesla. Looking at the battery pack, motor, and charge times, it's much closer to the i3 Coupe than the crude Mini E.

In July 2011 BMW unveiled the i3 and i8 electric vehicles in concept form. Remarkably little has changed about these vehicles as they reached production, other than leverage advances in battery cells, and other minor improvements.

The i3 refines the Active E's two-part battery pack, merging it into a single flat sheet in the tunnel region, protected by the aforementioned sheet of solid steel.

The 2014 BMW i3 Coupe EV [Image Source: Jason Mick/DailyTech LLC]

While the i3 Coupe has:

Vehicle Size

A 2,635 lb (1,195 kg) vehicle load

101-inch (257 cm) wheel base

Motor

170 hp

184 lb-ft of torque

0-60 mph 7.2 seconds

Rear-wheel drive

Battery

22 kWh battery

Refrigerant-gas cooled

Large-format cells

96 cells/8 modules (12 cells per module)

Charge Time

6 h at 240 V, 32 A

8-10 h at 240 V, 16 A

(tentative) range of 80-100 miles in "comfort" mode, 90-110 miles in "Eco Drive" mode

BMW's Head of North America Operation and Strategy further confirmed that this is actually BMW's third generation electric drivetrain, drawing from both the 1 Series and Mini lines.

The i3 is a true third generation electric drivetrain. If you must compare the most accurate comparison would be to say that i3 Coupe is somewhat like a riff on Mini in terms of the hat (body stylings), but in terms of the base is an improved version of the 1-Series electric drivetrain (from the ActiveE).

Major design improvements over the Active E include the confinement of the battery to a tighter under-the-seat package, and a downsizing of the control electronics.

The refrigerant cooling is another new addition, and one that BMW says makes the vehicle safer. If the steel armor is punctured, the refrigerant gas will quickly escape leaving the vehicle undriveable. While Tesla does include flame-resistant layers between its cells, its liquid coolant leaks slower when punctured allowing drivers to foolhardily drive off in the injured vehicle.

Both BMW and Tesla use flame-retardants in their battery packaging and feature modular designs, which prevent fires from spreading.

VI. Third-Generation Electric Powertrain Aims for the Top

Mr. Harb bragged:

In terms of [Tesla's Model S] it's using technology similar to our Mini E. That's two generations behind... In terms of maximizing the benefits of an electric powertrain, I don't think anyone is close to us.

(tentative) range of 80-100 miles in "comfort" mode, 90-110 miles in "Eco Drive" mode

The Model S is clearly a performance beast with a 310 KW motor. By contrast, the i3 Coupe relies on a lighter 125 KW motor. The Tesla vehicle's battery is also 2.7 times the capacity of the BMW. But based on the specs you could argue that the BMW is equally efficient (or even more efficient) than the Model S, albeit with only two-fifths of the horsepower/torque and with about half the range.

More objectively, though, the Tesla, BMW, and Nissan Motor Comp., Ltd.'s (TYO:7201) Leaf EV are all arguably "third generation" powertrains as the LEAF saw a major bump in a 2013 model year and the original leaf was preceded by an internal test powertrain. Likewise Tesla's Model S is the third generation model, descended from the Roadster powertrain, which in turn was descended from early prototypes.

Tesla Roadster and Model S [Source: Tesla Motors]

Unsurprisingly, all three matured electric drivetrains share some common optimizations -- namely, putting the battery pack beneath the drivers and passenger's seats and module battery pack designs. Like the Leaf, the i3 Coupe uses a smaller battery to drive a lighter vehicle. But like the Model S it's rear-wheel driven (like the Model S), versus the front-wheel driven Leaf. Rear wheel drive is one reason why at lower speeds the i3 Coupe is able to feel somewhat sporty.

Nissan Leaf EV

The one area where the BMW i3 Coupe scores a clear win over Tesla and Nissan is in the safety of the battery pack.

But what about traditional crash test safety and recent criticism regarding the i3 Coupe? Let's look at what kind of ratings the BMW i3 Coupe has received thus far and what they mean.

The BMW i3 got 4 stars ratings in the Euro NCAP. Even a Nissan Leaf got 5 stars in the Euro NCAP (4 stars in NHTSA).

Let's dig into this criticism. First, the 2014 BMW i3 Coupe has not yet received a safety rating from the NHTSA. Don't believe me? Hop on over the to the NHTSA crash safety page and see for yourself.

Second the European crash test examines two things that the American crash tests don't -- pedestrian safety and safety assist. The primary reason that the BMW i3 was bumped down to 4 stars in the European crash test ratings is they scored low in each of the categories.

The Safety Assist category particularly hurt the i3 Coupe, which scored a 55 percent. Had it not been hit so hard in this category it almost surely would have achieved a five star overall rating. I'm basing this on General Motors Comp. (GM) who scored lower in every other category than the i3 Coupe, but managed to earn the extra star via a gleaming 86 percent rating in the Safety Assist rating.

Makes sense right? Well, not entirely. You see, the safety assist category is based on whether or not a vehicle checks off a series of features many of which fall under the "safety nanny" category, such as speed limiters and seatbelt warnings.

The i3 was knocked points for lacking a seatbelt warning, which I suppose is slightly bad, but really bears no impact on how a belted passenger is affected by a crash. More ridiculously, the i3 was knocked points for not having a speed limiter. Having driven the i3, I really don't think that was a valid critique. After all, the car is pretty much inherently speed limited to a top speed of 80 miles per hour or so (in comfort mode, 70 mph or so in "eco" mode).

VIII. Europe EV Crash Tests -- Child Safety

Let's assume that most buyers are responsible and are only secondarily concerned about pedestrian safety -- their top priority is safeguarding their friends and loved ones in the vehicle. Thus the meat of the Euro NCAP ratings is the adult and passenger ratings. Here's how various models stack up:

Clearly the BMW was a bit behind, but it was close enough that it's worth digging into what the crash tests actually said. Let's first compare the Volt, the Leaf, and the BMW i3 Coupe in the child safety category:

So clearly the i3 Coupe and Leaf score better in child safety than the Volt due to actual crash performance. Both the i3 Coupe and the Leaf score the full 40 points, indicating more or less near-perfect performance inc crashes, easy installation, and sufficient instrucitons. But the EU regulators award Nissan two more percentage points -- what gives?

The regulators write:

A disabling system is available as an option for the front passenger airbag, allowing a rearward-facing child restraint to be used in that seating position. However, the information provided to the driver regarding the status of the airbag is not sufficiently clear. The risks of using a rearward-facing seat in the front passenger seat without first disabling the airbag are clearly indicated in the car.

In other words the only "safety" flaw in terms of child safety in the i3 Coupe is it doesn't have an airbag status indicator, even if you can turn of the airbag. So that must be where the BMW gets docked. Actually no; regulators write for the Leaf:

The passenger airbag can be disabled to allow a rearward facing child restraint to be used in that seating position. However, information provided to the driver about the status of the airbag is not sufficiently clear. Permanently attached labels clearly warn of the dangers of using a rearward facing child seat in that seat without first having disabled the airbag.

At first I guessed from the descriptions that EU regulators didn't like that customers could remove the label regarding the airbag disable seat.

But looking at the top performer (the Volvo V60 PHEV) it gets the same score as the Leaf -- despite performing a fraction below the top rating in one of the two major crash tests:
And the regulators write:

The passenger airbag can be disabled by means of a switch. However, information provided to the driver regarding the status of the airbag is not sufficiently clear. The dangers of using a rearward-facing restraint in that seat without first disabling the airbag are clearly indicated on labels in the car.

So so much for the "permanently attached" hypothesis. As far as I can tell, it appears that EU regulators after concluding that the Leaf and i3 Coupe were equivalent in crash tests decided to arbitrarily assign more points to Nissan and less to BMW. And both BMW and Nissan should be irritated by Volvo's rating, when the actual science tests showed the Volvo was the inferior member of the trio.

Had points actually logically followed the performance, the ratings become:

Volvo V60 PHEV --173

Nissan Leaf ------ 172

Toyota Prius ----- 170

BMW i3 Coupe --- 169

Renault` ZOE ---- 169

Chevy Volt --------163

Peugeot iOn* ----- 151

Given the problematic nature of the child crash ratings, let's move on to the adult crash test ratings.

IX. Europe EV Crash Tests -- Adult Safety

Here's the adult crash test results for the first (Volvo V60 PHEV) and second (Nisan Leaf) placed peformers, plus the i3 Coupe, and the Volt:

Compared the Volvo V60 PHEV, which is significantly better than the rest in terms of rear impact whiplash, the rest are in a virtual tie (with the Leaf/Volt at 2.9 and the i3 Coupe at a 2.8).

The Frontal impact is where things get interesting. On a six part model the Volvo V60 is again ahead, scoring at worst 6 "adequate" ratings (on three parts of driver and passenger dummies, a piece). The BMW appears the next best -- it has only one marginal part (the driver's leg) and two adequates. It actually receives more "Good" (the best rating) than the Volvo V60.

By contrast the Chevy Volt received 4 marginals and 4 adequates, while the Leaf received 3 marginals and 1 adequate. So they should get worse ratings than the BMW right? Well, the Volt does. But mysteriously the Leaf is awarded a tie (13.6 points) with the Chevrolet Volt. I'd expect something more along the lines of a 12.5 for the Leaf and a 13.5 for the i3 Coupe.

The i3 Coupe reportedly had "weak" protection for the driver's chest when it came to side pole impacts. This is a relatively rare type of crash, but let's just leave that rating as is.

X. Conclusions From Review of the EU Crash Data Reports

With the adjustments based on the number of "marginal" ratings in the frontal impact rating we get:

Volvo V60 PHEV --173

Nissan Leaf ------ 171

BMW i3 Coupe --- 170

Toyota Prius ----- 170

Renault` ZOE ---- 169

Chevy Volt --------163

Peugeot iOn* ----- 151

Now, I admit there's a flaw you might have noticed in my methodology. I only tweaked the Volvo, Nissan, BMW, and Chevy ratings. The Toyota, Renault, and Peugeot might similar shift. But at the very least based on the publicly available data on the four models we examined, the i3 Coupe looks to have perhaps unfairly received the short end of the stick.

That's not to say the i3 Coupe is perfect from the information we've seen thus far, safety-test wise. It appears to have a weakness to pole crashes.

But also be aware that the EU pole test is performed by carrying the car at 18 mph towards a rigid pole, which is struck at a 90 degree angle [source]. The U.S. crash test is arguably far more relevant to the most common kinds of pole collisions, angling the car at 75 degrees [source].

At worst I'd guess the i3 Coupe will get one four-star rating in either the side or frontal impact NHTSA tests. It's almost certain to score a five star score on the rollover tests. So if I had to make an educated guess, I'll guess that it will earn a five star rating, but not the "perfect" five star rating the Model S received.

But ultimately, regulators decision to rough up the i3 Coupe in the rankings also demonstrates the shortcomings of current safety tests for battery vehicles
.

Road debris can be deadly to a poorly protected EV like the Model S. [Image Source: HBarber]

Road debris collisions are arguably at least as common as pole collisions. With lithium ion batteries, this typically irritating form of accident can become a serious safety risk. Compared to the air-cooled Leaf or the alumnium-armored Model S, the i3 Coupe would likely score far better if battery safety were considered.